These studies were undertaken to address paradoxical observations which suggest that aldose reductase-linked stress (i.e. an increased ratio of NADH/NAD+ manifested by an increased lactate/pyruvate ratio like that associated with hypoxia) in diabetic and galactosemic rats is induced by different mechanisms. Retinas from normal rats and rats given an inhibitor of sorbitol dehydrogenase (SDI), CP-166,572 (50 mg/kg bwt/day), 2-hydroxylmethyl-4-(N,N-dimethylsulfamoyl-piperazino) pyrimidine, for 3 days were quickly removed and incubated 2 h at 37 degrees C in Krebs Herpes buffer containing sugars and alcohols as follows: 30 mM glucose or 5 mM glucose +25 mM galactose, sorbitol, or galactitol. Metabolites were measured in extracts of retinas and medium. 30 mM glucose, 25 mM galactose, and 25 mM sorbitol (but not galactitol) increased L/P ratios from 22+5 SD in 5 mM glucose to 35+5, 30-5, and 36+5, respectively (p<0.001 for each vs 5 mM glucose) and were 23+4 in galactitol. Lactate production was increased by 30 mM glucose (33+3 nmol/ug DNA, vs 26+3 for 5 mM glucose; p,0.0002) but not by galactose (25+3), sorbitol (26+4), or galactitol (24+3). SDI decreased L/P ratios of retinas in 30 mM glucose to 31+3 (p<0.001) but had no effect on lactate production. In retinas incubated in 25 mM sorbitol, SDI decreased L/P ratios to 28+4 (p<0.0003) without affecting lactate production (25+4). SDI had no effect on L/P ratios or lactate production by retinas in 25 mM galactose. These data indicate that elevated levels of glucose, sorbitol, and galactose (but not galactitol) cause reductive stress which is associated with increased lactate production by 30 mM glucose but not by 25 mM sorbitol and galactose. These observations support the hypothesis that reductive stress,induced by different mechanisms in galactosemic and diabetic rats, mediates vascular and neural dysfunction in these animal models.